Studies look at why some bugs make it in U.S., while others don't

University of Illinois Professor Andy Suarez talks about using historic museum ant collections to study the entry of foreign insects into the United States.

Argentine ants slipped into the country around 1891 at New Orleans, maybe in the soil of ship ballast or on a load of coffee or sugar cane, and quickly spread through the U.S. Southwest and California.

Today, they're what University of Illinois Professor Andy Suarez describes as a huge agricultural problem, although you don't hear as much about them as their relative the fire ant, another foreign invader.

The Argentine ants have good reason for playing their protective role. Those pests, in essence, suck the sugar from plants and secrete it as honeydew, a major ant food source, Suarez said recently.

The Argentine ant could be the poster insect for a new study by Suarez, a UI entomology and animal biology professor, and colleagues indicating that it isn't just the number of foreign insects arriving at the nation's doorstep, and the number of times they arrive, that determine whether an invader takes hold here.

"It's a combination," said Suarez, whose research, besides ant ecology and behavior, focuses on the process of biological invasion, among other things. "Opportunity alone isn't enough."

The accommodations they find when they arrive also appear to be a key factor.

No, they don't demand a five-star hotel. But the invaders have a harder time establishing themselves depending on their nesting habits.

If they rely on a certain kind of tree from their homeland, for example, and that tree isn't available where they come into the country, they're less likely to settle down and raise one family after another.

"A lot of arboreal nesting species of ants have very specific requirements," Suarez said.

Ground nesting ants don't have that problem, and Argentine ants are nesting site generalists that will nest anywhere, he said.

The issue of how invasive insects become established is important because their spread can have economic and environmental consequences. Fire ants, which damage crops and attack wildlife as well as people, tree-destroying Asian longhorn beetles and Africanized bees are all relatively recent examples. Illinois farmers have been concerned the last five years about invading Chinese soybean aphids.

"There are some unbelievably costly invasive insects," Suarez said.

Another invader common around here, albeit more benign than the Chinese soybean aphid, is the pavement ant, which originally came from Europe and can often be seen nesting in local sidewalks.

Meanwhile, ants in Hawaii, which had none before they tagged along with humans visiting the islands, do major damage to native plants and animals.

The study by Suarez and Phillip Ward, a professor at the University of California-Davis, also is a cautionary tale about the benefits of the nation's historic museum collections, which sometimes have a hard time finding money even for routine maintenance these days.

The two spent years looking at the ant collections at the Smithsonian's Institution's National Museum of Natural History. The ants were collected over the years at U.S. Department of Agriculture quarantine sites around the country, along with a log of their entry point and point of origin.

"The earliest record was from the 1920s so we were able to reconstruct the ants that have been hitting the U.S. border for much of the last century," Suarez said.

The ants had to be identified, not always easy, and their nesting habits researched.

Suarez and Ward were then able to make a statistical analysis looking at how the number of opportunities the invaders had to get in and their nesting preferences affected whether they had established themselves or not.

The study, published in the Proceedings of the National Academy of Sciences, was funded by the National Science Foundation.